As photosensitive materials for electrophotography, many organic semiconductors have been investigated. In particular, polyvinylcarbazole (PVCz) has been widely used for electrophotographic photosensitive elements for the reasons that PVCz is imparted with a photoconductivity by irradiation of the near ultraviolet light, greatly increases the electric conductivity by exposure to light, easily forms a film, and gives a good surface smoothness in the case of forming a photosensitive layer or film. In such an application, for spectrally sensitizing PVCz to increase its photoconductivity due to the visible light to a practical sensitivity for electrophotography, sensitizing dyes such as triphenylmethane-based dyes, pyrylium-based dyes, cyanine dyes, etc., are added to PVCz.
However, when PVCz having the increased, visible light-induced photoconductivity by a spectral sensitization using conventional sensitizing dyes is used for a photoconductive layer, the following problems may occur.
(1) In the case of a direct type electrophotographic process, since the photoconductive material layer containing the sensitizing dye is colored by the light absorption of the dye to cause background coloring, it is necessary to reduce the amount of the sensitizing dye as low as possible (which causes the reduction of the sensitivity) or to decolor the dye by heat or light after the formation of images (in this case, incomplete decoloring or instabilization to heat or light is liable to occur and hence the restrictions on preserving conditions are increased); and
(2) In the case of successive application of yellow, Magenta, cyan and shadow toners onto one photosensitive plate in a color reproducing system, the sensitivity of the photoconductive material layer to light images to be exposed is reduced by the shielding effect (filter effect) of each toner, whereby it is difficult to obtain a constant exposure condition.
Also, since the conventional sensitizing dyes absorb the visible light, they have no sensitivity to the wave lengths of the infrared region; or in order that the photoconductive material layer has a sensitivity to the wave lengths in the infrared region, a large amount of the sensitizing dye must be used. Therefore, it is difficult to manufacture laser printers using an inexpensive and high illuminance light source such as a semiconductor laser.